This invention relates to a printing device in which an idler gear in mesh with a gear coupled to a platen is displaced to engage with and disengage from a gear coupled to a tractor depending on the use of a cut-form or a serial-form sheet, and more particularly to a printing device of the above mentioned type in which, with the use of a serial-form sheet, an accidental rotational displacement of the gear coupled to the tractor occurring when the idler gear is temporarily disengaged from the tractor gear due to an operational mistake can be corrected.
In prior printing devices in which a cut-form sheet and a serial-form sheet are selectively used, the idler gear is disengaged from the gear on the tractor side to allow the platen only to be rotated when the cut-form sheet is used. If a serial-form sheet is to be used the tractor is engaged by operating a lever to engage the gear on the tractor side.
When, with the serial-form sheet in use, however, the lever is operated by mistake to temporarily disengage the idler gear from the gear on the tractor side, with the gears then being re-engaged to cope with the error, undesirable results will take place as follows.
As illustrated in FIG. 4(a), with both gears engaged together, the tooth surface of the idler gear X abuts against that of the gear Y on the tractor side in the paper feed direction indicated by Z, with no backlash in this direction Z. When the idler gear X is displaced to go out of engagement with the gear Y on the tractor side, the idler gear X is slightly rotated in the Z direction as illustrated in FIGS. 4(b) through 4(d) to press the tooth of the gear Y on the tractor side against the tension of the serial-form sheet. When the idler gear X is disengaged from the gear Y on the tractor side, the gear Y is returned to its initial position by means of the tension of the serial-form sheet. When these are then brought into re-engagement, the idler gear X at its tooth surface presses the tooth surface of the gear Y on the tractor side to angularly displace the gear Y by a degree corresponding to the tooth width in the direction opposite the paper feed direction Z.
Resuming operation with this state retained, the line spacing of the document becomes narrower than set at a place where the misoperation has taken place, resulting in a sloppy looking printed document.